EP Patent No 1422436 discloses a balance spring formed of silicon and coated with silicon dioxide to render the thermal coefficient substantially zero around the temperatures of COSC procedures, i.e. between +8 and −38° C. The silicon oxide is generally obtained by oxidising the silicon core which makes industrial manufacture of compensating balance springs possible.
However, other pairs of materials may be envisaged, but are more difficult to secure to each other and thus to produce.